The clouds are dark while protostars contract within them and glow in the infrared. Maybe some nearby stars excite the cloud enough for some hydrogen glow. Color of cloud is red or dark or perhaps some reflection from nearby stars.

The new stars ionize the cloud and make it glow red.

The new stars disperse the cloud and the regions change from red to blue in color.

The sequence above is perhaps illustrated by M8, M42, Rosette, to M45...

In some galaxies star forming regions appear blue. In others they appear red. Why is this? I thought star forming regions should all have the same characteristics of glowing hydrogen gas - red.

/Ira

Do you have some specific images in mind? Not all images are full visible light spectrum. If the emission nebula appears blue it probably is a B-band image taken at ~4400 angstroms. This would be a blue light image. The R-band is at 6400 angstroms and the I-band is at 8250 angstroms.

The colors we see with galaxy images are often the result of images that are not full-spectrum.

The clouds are dark while protostars contract within them and glow in the infrared. Maybe some nearby stars excite the cloud enough for some hydrogen glow. Color of cloud is red or dark or perhaps some reflection from nearby stars.

The new stars ionize the cloud and make it glow red.

The new stars disperse the cloud and the regions change from red to blue in color.

The sequence above is perhaps illustrated by M8, M42, Rosette, to M45...

Yeah - the reflection nebula of M45 is a different phenomenon than the emission nebula. But from what he is saying it sounds like he is talking about pictures of external galaxies in which sometimes the nebula appears pink/red and other times it appears blue. That is probably the result of the wavebands selected for the image. The blue light band is at 4400 angstroms and the red light band is at 6400 angstroms.

Good point about the wavelengths used for what are often 'false color' images.

I was being more simple-minded. Does the term star-forming region lump together several objects that evolve over a fairly short time compared to the age of the galaxy they reside in? Star forming regions could be red to blue if the terms lumps together:

1) objects that are mostly visible by emission from gas (like M42, maybe NGC 604? in M33, etc.) with

2) objects that are visible from both direct observation of hot blue starlight and the emission glow of the gas cloud (Rosette, Lagoon?), and with

3) objects that are young blue star clusters (there may be better or younger examples than Pleiades M45 that I used above, I wasn't meaning to refer to the dust reflection there but to the lack of emission glow).

It was a mistake by me to mention the Pleiades, I only meant them as young blue stars and instead the example distracted.

Here’s my answer for why star forming regions are red and blue:

I think “star forming region” is a process-based term.

The star forming process takes a very short time on a cosmic scale, millions to tens of millions of years. It starts with gas/dust/molecular clouds that have areas condensing and glowing in the infrared, then blazing as young, hot blue stars. The short-wavelength light and UV from the young stars makes the cloud glow red from ionized hydrogen, and then begins to ‘blow’ away the gas by radiation pressure and stellar wind from the young stars. As the gas recedes, the stars’ blue-white light becomes a greater aspect of the “star forming region” appearance. Finally, the young stars themselves disperse or scatter away from any remnants of the cloud. So the process-based “star forming region” has a short life on a galactic scale and rapidly changes from dim or invisible object with infrared peak, to visible hydrogen emission, to direct view of hot blue stars, all in the cosmically brief duration of the star forming process.

That’s my general recollection from reading that may well be decades out of date. Does it sound right now?

Jay Bird, your description of the sequence of events is still applicable, as far as we know. The red/blue aspect depends in part on age, as well as the mass/volume of gas which can be ionized. These aspects determine which source is (apparently) dominant; starlight or H-alpha emission. I think a study of a series of images--from very wide angle to Hubble--of the Tarantula nebula region in the Large Megellanic Cloud will offer insight into both the processes in action and the effect resolving power has on our perceptions.

In looking for images of blue and red star formation galaxies I found Wikepedia (of course) on star burst galaxies: One type is blue, low in dust; the other type is red/infrared, and is rich in dust. I think those must have been the galaxy types I was seeing images of.

The brightest stars in such regions are hot, young O- and B-types, hence the blue color. Emission nebulae--commonly associated with star forming regions--often feature strong peaks in Nitrogen II and Hydrogen-alpha emissions, hence their red appearance in digital images.

Bill in Flag

Exactly. More gas in an area, and it glows reddish (think Orion nebula). More stars and the area becomes bluer and bluer.
Given enough resolution, there is often a mix of pink and blue in every nebular complex. Look at Hubble pix of the NGC4038/4039 collision.

What had me confused is that the galaxies in these photos were either totally red or totally blue. Since star formation regions even in star burst galaxies must be rich in hydrogen gas which would glow red, how can there be star burst galaxies that are just blue?

What had me confused is that the galaxies in these photos were either totally red or totally blue. Since star formation regions even in star burst galaxies must be rich in hydrogen gas which would glow red, how can there be star burst galaxies that are just blue?

/ira

If distant enough, the red could be shifted into the infrared.
But then the blue would be shifted also, into the green or even yellow.
So the blue seen could be strong ultraviolet shifted into the blue.
If so, you would see blue only and no red.

You've see pictures of galactic fragments in Hubble photos colored yellow? Those are starburst galaxies and the un-shifted color would be ultraviolet.

What had me confused is that the galaxies in these photos were either totally red or totally blue. Since star formation regions even in star burst galaxies must be rich in hydrogen gas which would glow red, how can there be star burst galaxies that are just blue?

/ira

Radio telescope images often present galaxies as totally red. Images taken in the ultraviolet often present galaxies as totally blue.

It seems to me that, with all the processing that astro-photographs go thru and with the various color palettes used to represent data gathered outside of the visible spectrum, one has look at these images keeping in mind that they are not what an object "looks" like but are graphical representations of a wide variety of digital information.

sometimes I feel that there is so much to understand about Astronomy and the related sciences that its is very difficult for "laymen" to appreciate and comprehend all that is represented by astro-images and even the most user-friendly astro-news releases

is it possible that someday soon the forefront of science will require SO much background info and education that it will be beyond ourcomprehension??

It pretty much is, for someone who's not a specialist in the field. Heck, there are lots of aspects of geology that are incomprehensible to me, and I'm a geologist!

It means the job of synthesizers, those who interpret all this to the lay public -- and to policy makers and funders -- has become ever more important, and ever more difficult. It's a job that's getting harder to do well.

All that said, the general advantages of the scientific method, the scientific mindset, and the skill of critical thinking remain just as valid for all of us. The basic facts and the overview itself doesn't change that much over time. The details simply become more devilish.